2007 Vanguard dry van suspension question

Pretty sure it's built into the leveling valve. Do you have the support stands operated by a small air bag?

 

There might be a valve on the tank where the air fills the tank that diverts it to the parking brake and air bags first, then fills the tank after that. If the dump is integrated into the leveling valve, then you need to fill your air bags from the tank instead of from that primary valve before the tank. Then when you shut off the air supply to the tank, it will still have pressure from the tank.

 

It looks like the hose with black cable sheathing supplies air to the brake chamber that moves the rod, and also tees off into the yellow air line that goes into what I'm guessing is a pilot/dump port on the leveling valve. If you follow that black supply hose from the chamber, it probably goes to the spring brake priority valve or whatever it's called, probably attached to the other side of the tank. When you pull the knob, it cuts the supply to that hose, which shuts off the red/blue hose supply to the leveling valve and allows the air bags to exhaust to atmosphere.

I might be wrong about how it works, it's just a guess. If it's correct, then you have to find a way to get constant pressure to that dump/pilot port with the yellow hose. I would replace the coupler that joins the red and blue hose with a tee, then connect the yellow hose to it. Then you'll have to plug or replace the tee where the yellow hose used to connect at the chamber.

I don't know the effect it will have on the chamber and rod. If there is no way for that line to exhaust air, it will stay trapped in that line, which means the rod might not move back to its default position. Not sure if it matters, or if something can get broken if it's not in a certain position when the suspension dumps. It should exhaust like normal, but I'd make sure just in case it doesn't.